While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they often produce polymers with improved physical properties. However, in the production of linear low density polyethylene for film applications, especially in slurry processes, Ziegler-Natta catalysts predominate. One reason is that it has been difficult to use single-site catalysts to produce polyethylene with good film properties. While comonomer incorporation is usually good, the polyethylene often has poor impact properties or processes poorly because of its narrow molecular weight distribution.
Organometallic complexes that incorporate “indenoindolyl” ligands are known (see U.S. Pat. Nos. 6,232,260, 6,451,724, and 6,559,251 and PCT Int. Appl. WO 01/53360). In many of the known complexes, an indenoindolyl group is bridged to another group, which may be a second indenoindolyl group. The ligands are versatile because a wide variety of indanone and arylhydrazine precursors can be used to produce indenoindoles. Thus, substituent effects can be exploited and catalyst structure can be altered to produce polyolefins having a desirable balance of physical and mechanical properties. However, they have not been used in two slurry reaction zones to produce medium density or linear low density polyethylenes that provides good film properties.
Single-site catalysts containing an aryl group bonded to a Group 4 transition metal through an oxygen or sulfur atom used with an organoaluminum compound such as methyl alumoxane or a cation generator are disclosed in U.S. Pat. No. 6,323,149. The transition metal compound or the organoaluminum compound or both may be deposited on a support for use. These catalysts have not been used in two slurry reaction zones to produce medium density or linear low density polyethylenes with good film properties.
Multi-zone slurry polymerizations of ethylene with Ziegler-Natta catalysts are known. For example, U.S. Pat. No 4,357,448 discloses a two-step process for polymerizing ethylene in the presence of a Ziegler-Natta catalyst in combination with a reaction product of a titanium or vanadium halogen-containing compound with a first reaction product of a Grignard reagent with a hydropolysiloxane. A small amount of a second alpha-olefin is optionally used and the lowest reported density is 0.9515. The reference does not teach how to make medium density or linear low density polyethylenes with good film properties.
U.S. Pat. No. 6,486,270 discloses a process to polymerize ethylene with a C3-C10 alpha-olefin in the presence of high levels of hydrogen to make polyethylene with a density of from about 0.92 to about 0.94 g/cm3 with multiple reaction zones using a Ziegler-Natta catalyst. The polyethylene has improved film properties versus high-density polyethylene, but the film properties are insufficient for many applications. For example, when the polyethylene is blown “in-the-pocket” into a film having a thickness of 13 microns, the dart drop impact strength is less than about 50 grams (see Comparative Example 12, below).
There has been some use of single-site catalysts in two reaction zones. U.S. Pat. No. 6,566,450 discloses a process using bis-indenyl single-site catalysts to produce polyethylene with a bimodal molecular weight distribution having a density of from 0.95 to 0.96 g/cm3 useful as pipe resin. Not taught is how to make medium density or linear low density polyethylene with good film properties.
U.S. Pat. No. 6,552,150 discloses a process which polymerizes ethylene in two reaction zones to give polyethylene with a density of 0.929 to 0.934 g/cm3 and good film properties. A Ziegler-Natta catalyst is preferred. The preferred process is a slurry loop reactor followed by a gas phase reactor. While this process gives polyethylene with good properties, it would be desirable to produce polyethylene in an all-slurry process. A combination slurry and gas-phase process adds to complexity and cost. The reference teaches that “while it may be possible to use a process comprising cascaded slurry reactors only, such a process is not recommended, due to problems which may occur when a component having a low density is dissolved in the reaction diluent.”
Heretofore, it has been difficult to achieve low densities with Ziegler-Natta catalysts in two reaction zones in a slurry process. Because of poor comonomer incorporation, waxes build up and can foul the reactor. Single-site catalysts are known to give improved comonomer incorporation, but they often cannot achieve the required molecular weight due to competing chain termination reactions and decompositions which produce hydrogen. The resultant polyethylene has inferior film properties. In sum, there is a continuing need for an all-slurry process that can provide medium density or linear low density polyethylenes that give films with improved properties, especially impact strength and impact resistance.